We have developed a genetic system for expression and secretion of foreign antigens by the intracellular bacterium Listeria monocytogenes. Recombinant Listeria vaccine strains expressing the lymphocytic choriomeningitis virus (LCMV) nucleoprotein (NP), or a specific MHC class I restricted NP epitope, are able to induce LCMV specific CD8+ cytotoxic T lymphocyte (CTL) responses in mice following vaccination. These strains also confer antiviral protection as indicated by the ability of immunized mice to clear LCMV infection. Listeria strains that express the E1 protein of cottontail rabbit papillomavirus (CRPV) have also been constructed. Immunization of rabbits with these recombinants resulted in regression of CRPV induced papillomas and consequent protection from carcinoma. Our objective is to characterize the interaction between live L. monocytogenes vaccine strains and the immune system. We will focus on MHC class I antigen processing and presentation leading to the generation of CD8+ T cells. Studies with the LCMV model will identify parameters that are critical for efficient induction of protective cell mediated immunity. Information resulting from this proposal will provide the foundation for future efforts to use live, attenuated Listeria recombinants as novel tools for providing protection against intracellular pathogens and cancer. The specific aims are to: 1. Construct attenuated L. monocytogenes vaccine strains and determine their virulence and persistence characteristics. L. monocytogenes is human pathogen. Methods for attenuation that optimize immunogenicity and minimize virulence will be identified. 2. Construct L. monocytogenes vaccine strains with alterations in antigen expression level, antigen stability, and antigen localization. We will determine the most effective way to deliver antigen to the MHC class I processing and presentation pathway. 3. Determine the effects of attenuating mutations and alterations in antigen delivery on the induction of antiviral CTL. Primary CTL responses, CTL precursor frequencies and protection against LCMV will be measured. The duration and characteristics of CTL memory will also be determined. These and other assays will provide a quantitative assessment of CTL mediated immunity induced by vaccination with recombinant Listeria.